Waveguide applicator and method

ABSTRACT

A source of microwave energy excites a waveguide applicator which includes an input waveguide elbow, an output waveguide elbow and an elongated waveguide heating chamber coupled between the two elbows. Each of the elbows and the chamber are transversely defined by a pair of opposing broad walls and a pair of opposing sidewalls. The broad walls in the heating chamber of the applicator, excited in the TE10 mode, generate an electric field which decreases its angle of incidence with the material being treated as the electric field intensity decreases. This uniformly heats the material along its width. In a preferred embodiment, the top and bottom broad walls in the heating chamber are inclined symmetrically upwardly about the longitudinal center plane of the chamber so that the cross section of the heating chamber has a slight V-shape, and the angle of incidence of the electric field vector decreases away from the center of the chamber and toward the sidewalls. The material to be heated enters and exits the applicator through reject filters coupled to slots provided in the broad walls of the input and output elbows respectively. The reject filters prevent the escape of microwave energy through the entrance and exit locations without absorbing power. A water load is provided in the applicator downstream of the exit slot to absorb excess energy under varying loads; and means are provided to circulate air through the chamber to carry away moisture or cooking gases. A slot extends longitudinally along the center of the bottom broad wall. The slight V-shape of the heating chamber forms a natural trough for carrying fats, oils, etc. from the material being heated through the bottom slot under gravity flow.

United States Patent [72] Inventor RnyM..lohnson Danville, Calil. [21] Appl. No. 817,097 [22] Filed Apr. 17, 1969 [45] Patented Aug. 3, 1971 (73] Assignee Cryodry Corporation San Ramon, Calif.

[54] WAVEGUIDE APPLICATOR AND METHOD Primary Examiner-J. V. Truhe Assistant Examiner-L. 1'1. Bender Attorneys-Carl C. Batz and Dawson, Tilton, Fallon &

Lungmus ABSTRACT: A source of microwave energy excites a waveguide applicator which includes an input waveguide elbow, an output waveguide elbow and an elongated waveguide heating chamber coupled between the two elbows. Each of the elbows and the chamber are transversely defined by a pair of opposing broad walls and a pair of opposing sidewalls. The broad walls in the heating chamber of the applicator, excited in the TE mode, generate an electric field which decreases its angle of incidence with the material being treated as the electric field intensity decreases. This uniformly heats the material along its width. In a preferred embodiment, the top and bottom broad walls in the heating chamber are inclined symmetrically upwardly about the longitudinal center plane of the chamber so that the cross section of the heating chamber has a slight V-shape, and the angle of incidence of the electric field vector decreases away from the center of the chamber and toward the sidewalls. The material to be heated enters and exits the applicator through reject filters coupled to slots provided in the broad walls of the input and output elbows respectively. The reject filters prevent the escape of microwave energy through the entrance and exit locations without absorbing power. A water load is provided in the applicator downstream of the exit slot to absorb excess energy under varying loads; and means are provided to circulate air through the chamber to carry away moisture or cooking gases. A slot extends longitudinally along the center of the bottom broad wall. The slight V-shape of the heating chamber forms a natural trough for carrying fats, oils, etc. from the material being heated through the bottom slot under gravity flow.

PATENTED AUG 3 I9?! SHEET 3 OF 4 WAVEGUIDE APPLHCATUR AND METHOD BACKGROUND AND SUMMARY The present invention relates to a system for heating or cooking material with microwave energy; and more particularly, it relates to a microwave heating or cooking system wherein the applicator or oven which receives the material is a waveguide, as distinguished from a multimode cavity.

In the field of microwave technology a waveguide refers to a conduit, usually having a closed cross section for propagating and channeling electromagnetic energy in the microwave region. The term rectangular as used herein to characterize a waveguide means that there are two pairs of op posing walls (although not necessarily extending in the same plane or in parallel planes) bounding the waveguide on its sides. The symbol TE refers to the Transverse Electric field vector which is excited in the waveguide; and TM refers to the Transverse Magnetic field vector. When a waveguide is excited in the TE,,,,, mode,

I. there is substantially no electric field in the direction of power flow (herein referred to as the Z-direction),

2. there are m nodes in the electric field intensity profile in the direction parallel to the broadwalls (herein referred to as the X-direction), and

3. there are n nodes in the electric field intensity profile in the direction parallel to the sidewalls (herein referred to as the Y-direction).

Thus, the TE mode of operation in a waveguide refers to a condition wherein the electric field vector extends perpendicularly between the broadwalls (in the Y-direction), and there is substantially no electric field in the Z- or X-directions. It will be appreciated that the foregoing explanations, which have been for purposes of illustration, are theoretical. ln practice, these concepts are useful design tools only; and they are not to be taken as rigorous definitions of structure, operation or result. The free-space wavelength of the excitation frequency, k is the length of a period of the excitation frequency as it would exist if the wave were propagated through free space. The b dimension (which defines the perpendicular distance between opposing broadwalls) may be held at a value less than one-half the free-space wavelength of the excitation frequency to suppress the propagation of all TM modes and all TlE,,,,, modes where n is an integer greater than zero. Further, the a dimension (which defines the perpendicular distance between parallel sidewalls) may be maintained to be greater than one half of the free-space wavelength and less than one free-space wavelength to suppress TE,,,,, modes where m is greater than one.

In the TE mode, the intensity profile of the electric field varies according to a sinusoidal function from a minimum at one side of the treating zone through a maximum at the transverse center of the broadwall (i.e. x=z/2), and back to a minimum at the opposite side of the treating zone. Thus, for a given input power, the electric field intensity at the transverse center ofthe treating zone is at a maximum.

Since the waveguide heating chamber (which defines the treating zone) of the one illustrated embodiment of the present invention does not have a truly rectangular cross section, the term broadwall is given a somewhat broader significance. It encompasses those opposing walls in a waveguide to which the electric field intensity vector is perpendicularly incident.

An early development in the field of microwave or electronic cooking was the construction of a box having conductive sides (i.e. an electronic cavity) of generally rectangular shape and designed for heating food. The relative dimensions of wavelength and cavity at a given frequency are such that the cavity is several wavelengths on a side. Food material is placed in the cavity, the door closed, and the oven turned on. Mode stirrers are used in this even in an attempt to make the heat generation in the oven generally independent of the position of the material being heated. This type of oven-is some times referred to as a batch-type" oven since the material is heated or cooked in a batch; and the oven must be shut down for putting food into and taking food from the oven. Sometimes these ovens are referred to as multimode cavities because the large dimensions of the structure in terms of wavelength permits simultaneous resonance of many model patterns (i.e. modes). Reflectors and stirrers are usually provided in an effort to evenly distribute the multimode microwave energy.

A continuous-type microwave oven was developed to feed microwave power into an elongated metallic tunnel through a series of slit openings spaced longitudinally of the tunnel. These tunnels served the same function as multimode cavities since a number of different modes were excited and varied as the microwave energy entered the tunnel and by reflectors. A continuous belt conveyed the material being heated through the tunnel: and energy-absorbing devices in the form of water traps were provided at each end of the tunnel adjacent the entrance and exit openings for preventing the escape of radiation by absorbing it.

Another development included the use of a rectangular waveguide folded into a serpentine arrangement (sometimes referred to as a meander system) wherein the sidewalls of the waveguide remained in a common plane, but the broadwalls are formed to make multiple passes or folds across the intended path of the material being heated. That is, the guide is arranged so that the product passes through slots in the broadwall of the waveguide a number of times. Energy is piped into one end of the waveguide; and the energy level decreases along the direction of power flow finally terminating in a water load.

The advantages of the present invention may best be appreciated in terms of a specific application; and toward that end, the embodiment illustrated and described herein is particularly suitable in cooking large quantities of bacon in a relatively short time. It will be appreciated, however, that the invention is not limited to this particular application. In the packaging of bacon, the bellies are sliced at a high rate; and individual lots of convenience size are separately conveyed away from the slicing area.

Because of band restrictions, there are only two practical frequencies that may be used to cook the bacon with microwaves-2.4-5 l0 hertz (having a wavelength of 4.82 in.) and O.9l5 l0 hertz (having a wavelength of 12.9 in.). Assuming that latter is to be used, the length of a commercially available strip of bacon is about 12.0 inches. As mentioned, the power distribution in the X-Y plane of a rectangular waveguide excited in the TE mode is a sine-squared function, having a maximum at a/2 and reducing to zero at the sidewalls. lf the bacon were conveyed through a treating zone provided by a rectangular waveguide, then, with the direction of elongation of a strip extending transverse of the direction of ower flow through the guide-as is desirable because of the way in which the bacon is packaged-the center of each strip would receive most of the energy and the ends very little.

The present system provides a waveguide applicator having a pair of opposing broadwalls and a pair of opposing sidewalls. Power is coupled into the applicator by means of rectangular waveguides and an input elbow which feeds power to one end of the heating chamber. A second or output elbow is coupled to the other end of the heating chamber; and it feeds remnant power to a terminating water load.

The bacon is conveyed through the heating or cooking chamber in the direction of broadwall flow; and it is introduced through a broadwall in the input elbow which is designed to minimize the expenditure of power and to avoid exciting modes other than the TE modes where m is an odd integer. A reject filter is coupled to the input elbow adjacent the point of introduction of the bacon; and the bacon passes through the reject filter. The reject filter is designed to prevent the escape of microwave energy at the point of introduction of the bacon without absorbing such energy. A second reject filter is provided adjacent the location at which the bacon exits the output elbow.

Preferably, the upper and lower broadwalls of the cooking chamber are inclined upwardly symmetrical to the longitudinal center plane of the chamber so that in cross section, the heating chamber defines a slight V-shape. The term V- shape as used herein is not intended to limit the shape of the broadwalls of the heating chamber to planar or straight sections for, as will be realized upon a full understanding of the invention, the cross section of the broadwalls may be curved, although this would add cost to the illustrated embodiment. In this embodiment, the individual bacon strips are set on a flat conveyor so that the angle of incidence of the electric field vector between opposing broadwall sections is smaller at the periphery of the bacon strips where the electric field intensity is less than it is at the center of the applicator. That is to say, the electric field lines are perpendicular to the plane of the bacon at the transverse center of the treating zone; and they vary from the normal toward the sides of the treating zone. The angle of incidence, 6, of the electric field vector as used herein refers to the smaller of the two angles made between the electric field vector and a tangent to the surface of the material being treated at the location of incidence and extending in the X-Y plane. This arrangement has been found to provide a more uniform heating of the bacon substantially throughout the longitudinal direction (i.e. transverse of the direction of power flow), despite that wide variation in electric field intensity. This, of course, is highly desirable in the high-speed processing of bacon. The dimension b (which in the case of the V-shaped guide is the perpendicular distance between the planes tangent to the interior of opposing broadwall surfaces of the guide) is maintained less than one-half the wavelength of the excitation frequency in order to prevent propagation of all TM modes and those TE,,,, modes where n is greater than zero.

The downstream," end of the output elbow is provided with a water load termination which serves to absorb all of the microwave energy propagated through the heating chamber which is not absorbed by the material being heated. This water load is an electrical termination in the sense that little or no energy is reflected from it back into the heating chamber. The elbow sections at the point at which the bacon is introduced are specially designed with two criteria in mind. First, the additional modes of propagation that are excited are limited to TE,,,,, modes where m is odd while retaining most of the energy in the TE mode. Secondly, at the point of introduction of the bacon, the applicator is designed so that the electric field is substantially perpendicular to the surface of bacon in order to prevent excess dissipation of energy at this point, as more fully explained later.

Conduits are provided for coupling air into and circulating air through the heating chamber in exhausting moisture and gases produced during cooking or heating. A slot is also formed in the longitudinal center of the lower broadwall in the heating chamber. With the curved lower broadwall forming a trough, fat rendered by cooking the bacon is conveniently drained by gravity flow through this slot and disposed of. The entire applicator is designed so that it is excited symmetrically relative to the longitudinal center plane so as to prevent the excitation of TE,,,,, modes of propagation where m is an even number. Since the drain slot is located at the position a/2, TE,,,,, modes where m is an odd integer will not radiate through this" slot. The input and output elbows as well as the heating chamber are formed in two separate sides with center abutting flanges for conductively holding them together in operation. The sides may easily be separated for cleaning or maintaining the interior portions of the elbows and heating chamber for the entire applicator is separated along its longitudinal center plane.

The applicator is designed for coupling a predetermined amount of power to the material because power supplies are usually provided in predetermined power levels, a series of applicators may be conveniently arranged in tandem with the bacon successively passing through any number of modular applicators.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.

THE DRAWING FIG. I is a perspective view of applicators arranged in tandem;

FIG. 2 is a view in cross section of the input air coupling;

FIG. 3 is a side perspective view of one of the applicators shown in FIG. ll;

FIG. 4 is an interior perspective view of one side of a waveguide elbow and a reject filter;

FIG. 5 is a side perspective view of the interior'of the left side of an applicator shown in FIG. 1;

FIG. 6 is a side elevation view of the interior of the applicator section of FIG. 5;

FIG. 7 is a perspective view of the exterior of an input elbow and reject filter of an applicator;

FIG. 8 is an end elevation view of an input elbow for the applicators of FIG. 1;

FIG. 9 is a transverse cross-sectional view of the applicators of FIG. I; and

FIGS. 10--l2 are schematic tutorial diagrams illustrating a proposed, but not essential, theory which accounts for the improved results achieved by the present invention.

DETAILED DESCRIPTION As already mentioned, the illustrated embodiment is particularly suited for the application of microwave energy in cooking bacon; but persons skilled in the art will readily appreciate that the invention is not limited to that particular application.

Turning then to FIG. 1, there are shown four separate applicator sections generally designated 110, 11, 12 and 13 which are arranged in tandem and formed of a metallic conductor such as aluminum. For this particular application, convenience lots of bacon 15, with the strips in each lot being arranged in shingled fashion, are carried by means of a conveyor 16 through all of the applicator sections 1013. Applicator sections 10 and 11 are energized by means of a source of microwave energy generally 1 designated 17; and applicator sections 12 and 13 are energized by means of a source generally designated 18. Each of the power sources 17 and 18 may, for example, be capable of delivering 50 kilowatts of powerthus 25 kilowatts of power would be coupled to each of the applicator sections l013.

The sources of microwave pr: wer for the contemplated power levels will, in all likelihood, be magnetrons. These tubes have an efficiency which is highly dependent on the operating frequency for which they were designed. Any large shift in the load impedance will shift the operating frequency of the magnetrons and thus reduce their efficiency and shorten their useful lives.

Rectangular waveguides l9 and 19a couple power from the source 17 to the applicator sections 10 and 11 respectively. Aside from the differences in the waveguides l9 and 19A for coupling power to the individual applicator sections, each of these sections is similar in structure and operation so that only one such section need be explained in further detail for a complete understanding of the invention.

With particular attention, then, to applicator section 10, it includes an input elbow 20, a heating chamber (or treating zone) 21, and an output elbow 22. A tapered input coupling 23 extends in a generally vertical direction to couple power from the waveguide 19 to the input elbow 20. A termination section 24 is connected to the output elbow 22; and the termination 24 also extends in a generally vertical direction. The input tapered section 23 and the termination 24 may be considered as part of the applicator section 10.

Each section of the applicator is divided into side halves which are symmetrical about the longitudinal center plane (i.e. extending along the direction of power flow); and each complementary half section is provided with a flange for holding the halves in place. In practice, these flanges need not necessarily make a complete continuous conductivecontact because even if there is a slight separation, no appreciable amount of radiation will escape.

A conduit 25 extends vertically through the termination section 24 of the applicator in three separate passes in the illustrated embodiment, although more or less passes may be used. Water from a source not shown is forced through the conduit 25 to provide a load and absorb whatever power reaches the termination section 24. It is to be emphasized that the applicator 110 is a waveguide and that it is designed so that most of the energy is in the TE mode. Power flows from the source 17 through the waveguide 19, the tapered input section 23, the elbow 20, the heating chamber 21 and the output elbow 22 into the termination section 24 in that order. Further, the termination section 24 is a true termination in that very little power is reflected back into the heating chamber 21 once it is fed into the termination section 24. Thus, the water load conducted through the conduit 25, which may be a thin dielectric tubing, absorbs substantially all of the power not utilized in the heating chamber 21. The combination of a waveguide applicator and a termination wherein the product passes through the applicator permits the system to operate with little or no shift in the load seen by the source even though the amount of material being treated may vary appreciably. The load seen by the magnetron remains stable; and its frequency of operation is relatively independent of variations in the amount of material being treated. Further, the waveguide applicator which provides the heating chamber or treating zone as well as the termination section may be oversize. That is, the effective distance between the sidewalls taken along a line passing transverse of the broadwalls and equidistant from them may be greater than one wavelength of the excitation frequency to reduce the initial heating rate and attenuation constant, as explained in my copending application for SYSTEM AND METHOD FOR HEATING MATERI- AL EMPLOYING OVERSIZE WAVEGUIDE APPLICA- TOR, Ser. No. 816,500, filed Apr. 16, 1969.

An input rejection filter generally designated 26, and described in greater detail within, is connected to the input elbow 2t); and the conveyor lb and lots 15 of bacon move through the input reject filter 26 and the input elbow 20 into the heating chamber or treating zone 21. An output reject filter 27 is also provided at the location on the output elbow 22 at which the bacon exits the applicator section it) and is conveyed to the adjacent applicator section llll.

An input air conduit 28 is coupled to one side of the input tapered section 23 by means of a coupling box 29; and at this location a plurality of apertures 38 (see FIGS. 2 and 5) are formed in the input tapered section 23 to permit air to be forced therethrough. The air is removed via a similar coupling box 3i and output air conduit 31 connected to the terminal section 24 which is similarly provided with a number of apertures. The size of the apertures permits the passage of air but is small enough to prevent radiation of energy for the excitation frequency (about one-fourth to one-half inch diameter).

An ON/OFF switch generally designated by reference numetal 32 is provided at the interface between the waveguide 19 and the tapered input section 23 for selectively either preventing the flow of power into the applicator section or permitting the microwave power to be transmitted to the applicator section without attenuation. Alternatively the ON/OFF switch 32 may be replaced by a choke joint with the advantage that it will permit the separation of the applicator side halves without having to unbolt the flanges of the input guide 19 and tapered input section 23.

Turning now to FIG. 2, the input air coupling structure is seen in greater detail. The output air coupling structure is similar. As seen therein, the input tapered section 23 comprises first and second broadwalls 34 and 35 and first and second opposing sidewalls 36 and 37 (seen in FIG. 1). The input tapered section as well as the remaining portions of the applicator 10 are provided in separateable left and right side sections flanged about a vertical plane extending through the longitudinal center of the heating chamber 21. The left and right" convention refers to the sides as seen looking downstream in the direction of power flow. This structure permits the applicator to be separated into two side parts for cleaning, as explained in greater detail within. Thus, the left side of the tapered section 23 including the broadwalls 34 and 35 as well as the sidewall 36 is provided with the apertures 38. The coupling box 29 sealingly encloses all of the apertures 38 so that air forced through the input conduit 28 into the coupling box 29 will enter the input tapered section 23 via the apertures 38. The apertures 38 may be formed so that the area surrounded by the coupling box 29 is 50 percent permeable. Still referring to FIG. 2, a thin dielectric membrane 39 is fitted in the rectangular waveguide 19 above the location of the air pressure 38 to preventthe conduction of air to the source 17 while permitting the unattenuated flow of power through the waveguide.

As can be seen in FIG. 2, the switch 32 is selectively movable in and out of position; and at the location at which it is installed, there is a slot 19b provided in one of the broadwalls of the waveguide 19. A tongue 32a fits into the slot 19b and is selectively movable into a first position in which the edge of the tongue is aligned with a broadwall and a second position in which the tongue extends across the width of the waveguide and acts as a termination to prevent flow of power therethrough. This is a safety feature to reduce hazards while the system is being cleaned or otherwise dismantled.

Similarly, the termination section 24 is provided with aper tures to which the air may be exited into the output air conduit 31. The termination section 24 is fitted with an end plate24a (FIG. 1) which serves not only as a final termination for the termination section 24 but also as an air block for preventing the escape of exhaust air into the atmosphere surrounding-the applicator.

Referring to FIG. 5, one-half of the input taper section 23 is seen in greater detail to include a generally vertical flange 34a projecting outwardly of the left side of the broadwall 34 and a flange 35a projecting perpendicularly outwardly of the lefthalf section of the broadwall 35.

At the base of the input taper section 23 is the elbow 20 which couples power from the input taper section 23 into the portion of applicator section 10 which forms the heating chamber. The input elbow 20 is formed in left and right sections held together by center flanges as are the taper section, the output elbow, the heating chamber 21, and the termination section 24. As viewed again in the direction of power flow, the left-half portion of the applicator section 21 is supported by and fixed to the horizontal crossbars of a pair of metal horses 41 and 42. Similarly, the left-half portion of the input filter section 26 is supported on a metal horse 43. As seen in FIG. 5, the left-half side of the input elbow 20 is secured to a crossbar 41a of the horse 41 by means of nut and bolt assemblies 46. On the other hand, the right-hand section of the applicator is movably supported by the crossbars 41a and 42a to permit separation of the two sections of the applicator for cleaning.

Referring to FIGS. 5 and 6, the input elbow section 20 is provided with an upper flange 20a to which the input taper section 23 is removably secured by means of bolts 48. The

' input elbow 20 forms a smoothly curved transition for the A flange 52 provided about the aperture 51 provides a location at which the input filter section 26 may be secured. It will be appreciated from FIG. s that the disposition of the belt to is generally horizontal as it traverses longitudinally the entire treating zone; and the bacon or product to be treated is transported into the input elbow 243 at a location very near the bottom plate 50 thereof. This has been found to be of significant value in preventing the excess dissipation of energy at the point of entry of the product; and the smooth, symmetrical formation and transformation of the input elbow 2% has been found advantageous in preventing excessive excitation of undesirable modes, that is, TE, modes where m is greater than one.

Referring now to FIGS. 5, a, and 9, the heating chamber 21 is also divided into leftand right-hand sections along a vertical plane extending through the centers of its broadwalls. The left-hand section is seen in FIGS. 5 and 6; and the right-hand section is the mirror image of the left-hand section, so it need not be described in as much detail because description mode in connection with one side is to be taken as applying to the corresponding portion of the complementary section. The heating chamber 21 is elongated in the horizontal direction and its broadwalls (in the sense defined above) generate an electric field which is confined to a'vertical plane perpendicular to the horizontal flow of power.

The left-hand section includes upper and lower parallel walls 53 and 54 respectively. The upper wall of the left-hand section Zlla is provided with a vertical flange 55 which engages a corresponding flange 56 located on an upper wall 57 of the right hand member Zlb of the heating chamber (see FIG. 9). The upper and lower walls 53 and 54 are in parallel relation as can be seen in FIG. 9 although in its broader aspects the invention need not be so limited. That is to say, the walls 53 and 54 may be curved as viewed in cross section transverse of the direction of power flow. These walls are joined at their far end by means of a perpendicular sidewall 53. Similarly, a lower wall 59 on the right side Zlb is formed parallel to the upper wall 57 thereof; and a right sidewall as extends perpendicularly between the upper and lower walls 57 and 59.

The upper walls 53 and 5d are inclined so as to make an angle of about with a horizontal plane passing through their line of intersect. Since the lower walls 54 and 59 are parallel respectively to the upper walls 53 and 57, the lower walls also form an angle of about 140 relative to each other. The lower wall 54 is provided with a flange extending in a vertical plane 61; and the lower right wall 59 is provided with a similar flange 62. However, the flanges 6i and 62 do not ongage each other, but rather are separated to form a slot 63 through which fat rendered from the bacon during cooking may drain.

The inclination of the lower walls 5 5 and 59 provides a trough through which the rendered fat may conveniently drain and collect at the slot 63; however, as will be discussed later, this is not the primary function for so inclining the walls 54 and 59.

Referring particularly to MG. 6 and the interface between the input elbow 2t and the heating chamber 21, the end of the heating chamber 21 which connects to the downstream end (also referring to the direction of power flow) of the input elbow 20 is modified to adapt the contour of the elbow to that of the slight V-shape of the major portion of the heating chamber 21. Referring nowto both H68. 5 and t5, the bottom plate 50 of the input elbow 2t connects with an inclined plate of which (as seen in FIG. ti) is perpendicular to the plane of the page. The plate 68 has a triangular shape; and a second plate 69 connects the forward or upstream end of the bottom wall 54 ofthe left-halfsection 21a of the heating chamber with the plate 68 to form a continuous bottom for that side of the heating chamber.

It will be observed that the intersection between the plates 68 and 69 forms a corner drain or trough leading directly to the fo ard edge of the slot as for draining purposes. The other four lower corners of the heating chamber are similarly modified to interface with the elbows. Thus, the bottom of the heating chamber is inclined such that the entire area is drained of liquid under force of gravity.

A complete flange interface is provided between the elbow 2t) and the heating chamber 21 as at 70 in FIG. 6.

The inclined sidewall 58 of the heating chamber Zll is also modified to interface with the vertical sidewall of the input elbow. This includes a single plate which has a lower section 71 (see FIG. 6) integral with the outboard edge of the bottom plate 69 and the downstream edge of the sidewall of the input elbow 2t), and an inclined portion 72 integral with the plate 71 and the sidewall 53 of the midsection of the heating chamber 21. The sections 71 and 72 may be formed out of a common plate by bending along the fold line 73; and this is more clearly illustrated in FIG. 3 for the corresponding opposite plate which is designated by reference numeral 75 generally, and includes an upper portion 75a which lies in the plane of the inclined sidewall 60 of the heating chamber and a generally vertical portion 75b which forms a continuation of the vertical sidewall 20c of the input elbow 20.

Turning now to the downstream section of the heating chamber 21 which interfaces with the output elbow 22, this portion of the heating chamber, as illustrated in FIG. 6, has counterpart elements for each of the elements just described in connection with the interface between the heating chamber and the input elbow 20. Thus, the output elbow 22 includes a bottom plate 78 over which the belt 16 passes; and an inclined drain is formed by plates 81 and 82 which leads to the downstream end of the slot 63. As has already been mentioned, the right-half section of the input and output elbows as well as the heating chamber are symmetrical with respect to the left-half member relative to a vertical plane passing through the center.

Turning now to the input rejection filter 26, it, too, is designed in two separatable halvesa right-half 26a as seen from the inside in FIG. 4, and a left-half 2617 as seen in FIG. 7. Again, the right-half of the input rejection filter forms a mirror image of the left-half of this filter; and only the left-half as seen in FIG. 7 need be more fully detailed in order to understand this portion of the as will be appreciated'from FIG. 4, the right-half of the rejection filter is provided with a bottom plate M which forms the continuation of the bottom 78 of the righthalf of the input elbow 22. The left-half 26b ofthe input rejection filter is similarly provided with a bottom plate 85 which forms a continuation of the bottom plate 50 of the left-half side ofthe input elbow 20.

The input rejection filter includes three rejection sections formed in the upper broadwall each designated 86; and these sections are designed to completely reject the frequency which excites the wave guide applicator. A separate filter section 37 interfaces between the rejection sections 86 and the input elbow 22 in order to match the junction at the input elbow and thereby minimize reflections. The combined transverse length of each of the rejection stubs 86 (that is, the dimension transverse of the direction of movement of the bacon) is less than one and one-half times the wavelength of the excitation frequency. The three filter sections 86 are periodic; and the period of the stubs (that is, in the direction of movement of the bacon) is one-half the guide wavelength. The height and width of each of the stubs 86 is a quarterwavelength of the guide. In matching impedance's by means of the matching section 87, the height of the section 87 and its displacement from the point at which the bacon enters the input elbow 20 are adjusted in order to minimize the reflections seen at the source. This is best done empirically; and it depends upon the wavelength of the guide, the frequency of excitation and the input impedance of the rejection filter 26. Similar design considerations are given to the output rejection filter 27; and it will be appreciated that each waveguide applicator section has both an input rejection filter and an output rejection filter.

It will be observed that the reject filter stubs are placed in the upper broadwalls of the input and output filter sections.

This has two advantages. First, it avoids draining of rendered liquid into the cuplike stubs. Secondly, the electric field vector is constrained to be normal to the product because of the flat bottom wall 85 and the proximity of the product to that wall.

Turning now to the termination section 24l, it again is formed with symmetrical rightand left-hand sides which are held together by means of abutting center flanges. The previously mentioned conduit or hose 25 for carrying the water load extends within the waveguide section to absorb energy which has not been absorbed by the bacon.

Before discussing the particular advantages of the embodiment which has been illustrated, some of the theoretical background will be given in order to facilitate understanding of the overall operation of the inventive system.

In a rectangular waveguide structure, the width, 0, of the broadwall may be constrained to be less than the free-space wavelength of the excitation frequency and the width of the sidewalls to less than or equal to one-half a in order to suppress modes other than the TE mode. As disclosed in my copending, co-owned application for SYSTEM AND METHOD FOR HEATING MATERIAL EMPLOYING OVERSIZIE WAVEGUIDE APPLICATGR, identified above, the dimension, a, may be increased with certain advantages provided the restriction on dimension, b, is maintained and care is taken in the way in which the waveguide is excited. Such an oversize waveguide having a rectangular cross section excited in the TE mode, has an electric field with only a vertical (or Y-direction) component that is constant; and the electric field intensity varies in the X-direction according to a sinusoidal variation wherein the electric field intensity is zero at the sidewalls and a maximum in the center plane of the waveguide extending in the direction of power flow. If the waveguide is oversize, then the termination section supporting the water load would preferably also be oversize to prevent the accumulation of excessive amounts of reactive energy in the heating chamber of the applicator, as explained in the lastidentified eopending application.

Since the power coupled to any lossy dielectric placed in the field varies approximately as the square of the electric field intensity for constant frequency, the power coupled to the material varies as a sine-squared function according to the transverse position of the portion of the material being heated in the waveguide. Thus, for a thin, lossy material placed parallel to and between the broadwalls (that is, in the X-Z plane), the variation in heating rate results in nonuniform processing. If the material (for example the bacon of the illustrated embodiment) is centered at the vertical center plane of the waveguide and the width of the material is very much less than the width of the waveguide, then the degree of nonuniformity will not be significant; however, if the width of the material being treated is an appreciable portion of the overall width of the waveguide, the difference in power coupled to the incremental portions of the material will vary appreciably.

Although the waveguide applicator in the illustrated embodiment has a heating chamber which is not truly rectangular in cross section, nevertheless, the electric field intensity vector extends substantially perpendicularly between the parallel upper and lower walls 53 and 54 as well as the parallel upper and lower walls 57 and 59 (see FIG, 9), It is in this sense that the walls 53 and 57 form an upper broadwall" and the walls 54 and 59 form a lower broadwall." Further the amplitude of the electric field intensity vector is zero at the sidewall 58,

rises to a maximum in the vertical center plane, and then diminishes back to zero at the sidewall 60, although the exact variation may not be a sine wave as in the case ofa waveguide of a truly rectangular cross section. Thus, the electric field intensity along the length of one of the bacon strips (that is, in

, the X-direction) does vary. However, it has been found that by forming the waveguide as in the illustrated embodiment wherein the upper and lower halves of the broadwalls are inclined about the center plane and without changing the orientation of the bacon strips, the cooking of the bacon is substantially'more uniform throughout then would be expected from a truly rectangular waveguide of the same width.

sipated in the X-direction in a rectangular waveguide, the

power coupled to a thin product (that is, thin in relation to -the wavelength of the applied electric field, and having a depth for example of one-tenth of that wavelength) is a function of the orientation of its surface with respect to the electric field vector. A maximum coupling of microwave energy is brought about when the larger dimension (that is, the surface of the material) is parallel to the electric field vector. In other words, the thin" dimension extends in he X-direction. Minimum coupling is experienced when the surface of the material is perpendicular to the electric field vector.

Hence, for a thin material, the coupling of microwave power into the material passed through a waveguide excited in the TE, mode varies from a maximum coupling when the thin material extends in a plane parallel to the electric field vector to a minimum when the material lies in a plane perpendicular to the electric field vector. For orientations of the material in any other direction, the situation-may be represented as in FIG. lll wherein the material is designated by reference numeral 92 and the electric field vector by an arrow 93 extending in a vertical direction. It will be observed that the arrow has an angle of incidence relative to the material 92 which is perpendicular to a tangent to the material at the point of incidence. Hence, the power coupling into the material 92 is at a minimum at this point. Conversely, toward the right edge of the material 92 which slopesdownwardly, the electric field intensity vector E represented by an arrow 94 forms an angle 0 which is less than The vector E may be resolved into a normal component E',, and a parallel component E,,. In this case, the total power dissipated in the material 92 is the sum of the power dissipated by the electric field vectors E and E',,. In comparing these two examples, even though the electric field intensity is greater at the location of the vector 93 than it is at the location for the vector 94, nevertheless, because of the reduced angle of incidence (thereby inducing a parallel coupling component) the overall coupling becomes more equalized. As already indicated in referring to an angle of incidence, it will be observed that there are always two such angles in any plane; and as used herein, the phrase refers to the smallest of these two angles.

Referring now to FIG. 11, the sides of the waveguide providing the heating chamber 21 of the illustrated embodiment are schematically illustrated by the perimeter line 95; and the electric field intensity lines are indicated by the parallel dashed lines 97. It will be appreciated that the electric field lines 97 are only idealized representations of the actual field vector and that their orientation changes direction at the excitation frequency. The orientation of a single strip of bacon in cross section is designated 98, and the supporting surface (which may be the previously described conveyor belt) by reference numeral 99. It will be appreciated from FIG. 11 that whereas the angle of incidence of the electric field vector at the center of the bacon strip 98 is 90, the angle ofincidence 8 at the-outer extremes is somewhat less than 90. In a preferred embodiment, this angle is 70. The idealized illustration in FIG. Ill is only an approximation to a continual variation of the angle ofincidence 6 but one which nevertheless does vary the coupling from the point at which the electric field intensity is at a maximum intensity to a point at the edge ofthe material being treated where the electric field intensity is reduced.

An alternative for accomplishing like results is illustrated in FIG. 10 wherein the dashed line 100 represents the previously mentioned sine-squared energy dissipation curve; and the line 101 diagrammatically represents the rectangular cross section of a waveguide. In this example, the electric field vectors 102 are everywhere perpendicular to the planar broad wall; and the supporting surface 103 for the material 104 being treated is curved. Again, the angle 0 decreases from the point at which the electric field intensity is at a maximum to the peripheral edges of the material being treated where the intensity is reduced.

As has already been mentioned, the idealized diagrams of FIGS. 10 and 11 are only approximations in the sense that the angle of incidence does not continuously decrease throughout as one proceeds from the center to the sidewalls. However, the effect described herein may be combined with the disclosure of the above-identified application which describes the design of an oversize wave guide applicator to achieve a more even heat dissipation, if desired.

The angle 1 made between the horizontal and the broadwall portions of the V-shaped waveguide is illustrated in FIG. 11; and for bacon processing it is preferably about 20. It will be appreciated that not only may this angle be changed in order to effect different results, but that the angle 1 made vary in the Z-direction of the heating chamberso that, for example, toward the input power end, the angle 4 may be l5"; and at the downstream end, the angle b may be 25 in order to control heating to the peripheral edges of the material being treated as the function of Z. As mentioned, the term V- shaped" is not to be limited not is it to be inferred as meaning that the inclined edges of the broadwalls necessarily are straight since in order 'to control the heating or coupling rate under certain circumstances it may be necessary to curve these walls and to continuously decrease the angle of incidence as the electric field intensity decreases.

The perpendicular distance between opposing broadwalls (see b in FIG. 9) is kept at less than one-half wavelength of the excitation frequency so that even if the TE,,,,, modes (where n is an integer greater than zero) are excited, they will not propagate. The TE,,,,, modes where the bacon m is an odd integer will not In this through the slot 63 because they are symmetric relative to the center position of the slot. The TE, modes where m is an even integer are not excited because of the symmetrical excitation of the waveguide applicator.

Keeping in mind the difference in coupling power into the bacon as a function of the angle of incidence of the electric field, it will be appreciated that by introducing the bacon to the applicator along the bottom plate 50 of the input elbow (FIG. 6), the incident field is more nearly perpendicular than if the bacon were near the transverse edge 50a. in this way, excessive power dissipation in the material at the point of introduction is avoided.

Persons skilled in the art will also appreciate that the velocity of the traveling wave through the treating zone is very much greater than the velocity of the material being treated so that for all practical purposes, for a transverse of any given wave front through the treating zone, the product has moved very little and may be considered to be at a standstill.

Having thus described in detail one embodiment of my invention and indicating certain alternatives which will produce like-or similar results and having proposed a theory which may explain the improved results but which need not necessarily be the underlying theory which causes them, the advantages of my invention will now be enumerated.

The invention permits the use of a waveguide applicator in heating or cooking material. This prevents significant ad-v vantages in a number of areas, the most important being that it permits one to match impedances, insures the propagation of predetermined modes of energy through the waveguide, and (in combination with the water load) stabilizes the load on the source even though the amount of material being treated may vary. Further, a waveguide design and the limited aperture size permits the use and design of rejection filters at the locations of introduction and exciting of the material being treated without having to provide the inefficient water loads required in prior systems. That is, the energy seeking to escape is simply rejected without dissipation, whereas, in some prior systems, the escape of energy was prevented solely by absorbing all such energy at openings.

Another advantage is that in the use of a waveguide cooking applicator, the heating throughout the width of the product (that is, in the X-direction) passed between and in the direction of elongation of the broadwalls, is made uniform by decreasing the angle of incidence of electric field vector as the electric field intensity itself decreases.

Another advantage of the present invention is that it permits one to predetermine the heating or coupling not only in the X- direction for the material being treated but also in the Z- direction, as by continuously varying the angle 1 made between the horizontal and the broadwall of the heating chamber has a function of the Z-direction. The heating may be further controlled in the X-direction by curving the broadwalls in that direction while maintaining the bacon in a horizontal plane to continuously decrease the angle of incidence, 0, as the field intensity decreases.

Another advantage of the present invention is that the load may be varied without the danger of excess radiation into the atmosphere or excess reflection of the energy or excessive electric field buildup since the water load is a true termination and absorbs all energy not previously absorbed by the material being treated with little reflection. This simplifies the safety requirements and design monitoring of the applicator.

Still another advantage is the particular construction illustrated in which the separate halves of the waveguide applicator as well as the input and output elbow permit the dismantling of the machine for cleaning by separating into two sides, the interior of which is readily accessible for cleaning.

Still another advantage of the instant invention in the cooking or treating of material wherein fat, oil, or water is rendered during the process, is that it permits the provision of an elongated drainage slot without overcovering protection as in the case of multimode cavities. The slot may extend in the direction of movement of the product without the loss of energy through radiation. It will be appreciated that since the slot 63 is in the center of the lower broadwall and the only mode excited in the waveguide is the TE mode, the transverse current vector at the centerline of the broadwall is zero and energy will not be radiated.

Still another advantage is the provision of the input and output elbows and the cooperation therewith of the introduction of the bacon at a location wherein the electric field intensity vector is substantially normal to the surface of the material being treated. This is in order to obviate excessive coupling at a point where the field strength is at a maximum.

Still another advantage is the cooperative effect between the bottom of the slight V-shaped waveguide applicator and the slot which forms a natural gravity drain for all liquids rendered during treatment.

A still -further advantage is the ability to use commercially available modules of microwave power and arrange the microwave applicators in tandem according to the amount of heat required in processing.

Having thus described in detail a preferred embodiment of the present invention, it will be apparent to persons skilled in the art that certain of the structural elements described may be modified and that other elements may be substituted for performing similar functions without departing from the inventive principle; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the invention.

I claim:

1. A system for applying microwave energy to a material comprising waveguide means providing first and second spaced-apart broadwall means arranged in opposing relation, excitation means including a source of microwave energy coupled to said waveguide means for energizing the same to provide an electric field of varying intensity extending between said first and second broadwall means, supporting means for supporting said material between said broadwall means and means for moving said material between said first and second broadwalls and along the direction of power flow through said waveguide, said waveguide means and said supporting means cooperating to vary the angle of incidence of said electric field on said material as the intensity of said electric field varies to thereby couple energy into said material along said direction of varying field intensity according to a predetermined pattern.

2. The system of claim ll wherein said supporting means supports said material in a generally flat disposition and wherein at least one of said first and second broadwall'means defines a surface inclined relative to the disposition of said material in a direction transverse of the movement of said material.

3. The system of claim 2 wherein said waveguide means includes a waveguide-heating chamber defined on a first pair of opposing sides by said first and second broadwall means and on a second pair of opposing sides by first and second sidewall each coupled between said first and second broadwall means, and wherein said excitation means energizes said waveguide heating chamber substantially entirely in the TB mode.

4. The apparatus of claim 3 wherein said waveguide-heating chamber is elongated in a generally horizontal direction and each of said first and second broadwall means is symmetrically inclined about its longitudinal center to define a V-shape whereby the angle of incidence of the electric field lines between said broadwall means decreases as the intensity of said electric field decreases.

5. The apparatus of claim 3 wherein said waveguide means further comprises input waveguide means interposed between said heating chamber and said excitation means for coupling power therebetween, said input waveguide means further defining an input aperture for receiving said material and permitting passage thereof to said heating chamber, and rejection filter means coupled to said input waveguide means adjacent said opening to prevent the escape of microwave energy therethrough.

6. The system of claim 5 wherein the lower broadwall of said heating chamber is V-shaped in transverse section having a low point at its transverse center and defines a slot extending in the center thereof along the direction of power flow whereby liquids rendered from said material during the application of microwave energy thereto are drained under gravity along said lower broadwall toward said slot.

7. The system of claim 5 wherein said input means is a rectangular waveguide elbow-receiving power in a generally vertical direction and transmitting the same to said heating chamber in a generally horizontal direction, and wherein said aperture in said input means is defined in a broadwall thereof transverse to the flow of power therethrough.

8. The system of claim ll further comprising termination means coupled to said waveguide means downstream in the direction of power flow from a location at which said material moves between said first and second broadwalls whereby said termination means absorbs power not absorbed by said material to minimize reflected power.

9. The system of claim 8 wherein said broadwalls of said waveguide means provide a heating chamber, and further comprising output waveguide means interposed between said waveguide-heating chamber and said termination means for coupling power therebetween, said output waveguide means further defining an output aperture for exiting said material from said heating chamber, and rejection filter means coupled to said output waveguide means to prevent the escape of microwave energy through said opening.

it). The system of claim ll wherein said termination means comprises an elongated termination waveguide section receiving microwave energy from said broadwalls, and conduit means within said termination waveguide section and extending longitudinally thereoffor carrying water through the same.

ill. The system of claim ll wherein said waveguide means comprises an elongated waveguide-heating chamber of rectangular cross section extending in a generally horizontal direction whereby said broadwalls are planar, and wherein said supporting means moves said material longitudinally through said heating chamber and supports said material in a disposition angularly inclined relative to said broadwalls over atleast a portion of said material.

12. The system of claim 1 wherein said waveguide means includes input waveguide means receiving power from said source and including a waveguide input elbow delivering said power in a generally horizontal direction, waveguide-heating chamber means receiving power from said input waveguide means and having upper and lower opposing broadwalls each defininga V-shape in cross section with the center being the lowest point of each, and output waveguide means receiving power from said heating chamber and including means for absorbing said power in termination.

13. The system of claim 12 wherein said lower broadwall of said heating chamber broadwalls an elongated slot extending through its center in the direction of power flow.

M. The system of claim 13 wherein said input elbow defines an opening for admitting passage of said material to said heating chamber, and further comprising wave guide rejection filter means at said opening to prevent the escape of microwave energy therethrough.

15. The system of claim 12 wherein said side opposing section of said V-shaped broadwalls is planar and opposing pairs are separated by a distance less than one-half the free space wavelength of the excitation frequency.

16. ln a system for cooking separate lots of bacon strips of convenience size wherein the strips of each lot are in shingled relation, the combination comprising a waveguide applicator providing a heating chamber having upper and lower opposing broadwalls extending in a generally horizontal direction each longitudinal side of each of said broadwalls inclined upwardly about the longitudinal center line thereof, conveyor means supporting a plurality of lots of said bacon for carrying the same in a generally horizontal disposition through said heating chamber between said broadwalls with each individual strip extending transversely of said broadwalls, and source means for energizing said applicator substantially entirely in the TE mode.

17. The system of claim lfiwherein said applicator further comprises input elbow means receiving power from said source for coupling said power to said waveguide-heating chamber, said input elbow receiving said power in a generally vertical direction and transmitting the same in a generally horizontal direction to said heating chamber and defining an aperture for admitting passage of said bacon to said chamber.

18. The system of claim 17 further comprising rejection filter means adjacent said admitting aperture for preventing the passage of microwave energy therethrough.

19. The system of claim 16 wherein the lower broad wall of said heating chamber defines an elongated slot extending longitudinally along the center line thereof for draining fat rendered from-said bacon 20. The system of claim 19 further comprising means for circulating air through said heating chamber.

21. The system of claim 20 further comprising an output wave guide elbow receiving power from said heating chamber for transmitting the same in a generally vertical direction and defining an aperture for admitting the egress of said bacon, and termination means receiving power from said output elbow and including a load for absorbing substantially all of said power without reflection.

22. The system of claim 21 further comprising output rejection filter means coupled to output wave guide elbow adjacent said aperture for preventing the passage of said microwave energy therethrough.

23. The system of claim 19 wherein said heating chamber is symmetrical about a vertical plane extending through the longitudinal center thereofto prevent the excitation of TE, modes where m is an even integer.

24. The system of claim 23 wherein the symmetrical half sections of said heating chamber are separateable to permit access to the interior thereof for cleaning and maintenance.

25. ln a system for applying microwave energy of a given wavelength to a material, the combination of a waveguide, and source means for energizing said waveguide in the TE mode, said waveguide providing first and second opposing broadwalls elongated in the direction of power flow through, said waveguide and inclined relative to a surface ofsaid material to vary the angle of incidence of the electric field on said surface as said electric field decreases in a direction transverse of the direction of power flow through said waveguide to thereby cause a more uniform heating of said material along a direction transverse ofthe direction of power flow.

26. The system of claim wherein said material has a generally horizontal disposition, and wherein opposing portions of said first and second broadwalls are parallel to each other and inclined relative to the horizontal whereby one of said broadwalls beneath said material will catch any liquid rendered therefrom and cause the same to be collected.

27. in a system for heating material having a thickness which is generally uniform along a first direction, the combination of means for conveying said material in a second direction perpendicular to said first direction, and waveguide means having opposing broadwall means for generating an electric field varying in intensity in said first direction and for transmitting microwave power in said second direction, said waveguide means cooperating with said conveying means to orient said electric field lines to vary the angle of incidence of said lines on the surface of said material as said intensity varies to thereby cause a more uniform heating ofsaid material along said first direction even though the intensity of said field decreases along said first direction.

28. A method of heating a material having a generally uniform depth along one direction, comprising propagating microwave power through a waveguide in the direction of elongation of its opposing broadwalls, and passing said article between said broadwalls in the direction of elongation thereof while establishing the electric field intensity lines to vary their angles of incidence on the surface of said article in a plane perpendicular to said direction of elongation to induce uniform heating of said article along said one direction.

29. In a method of heating a material, the steps of passing said article in one direction through a waveguide, propagating microwave power through said waveguide along said direction, and establishing the electric field of said power to vary its angle of incidence on the surface of said article to cause uniform heating of said article along a second direction transverse of said one direction.

30. The method of claim 29 further comprising the step of passing air over said material as it is heated with said microwave energy.

31. The method of claim 29 further comprising continuously draining liquid rendered from said material during heatmg.

32. A method of heating a material having length, width and generally uniform depth comprising transmitting microwave power in a first direction through a treating zone, said microwave power generated by an electric field excited at a predetermined frequency and the intensity of said field varying from a minimum at the side of said treating zone to a maximum intermediate said sides, moving said material through said treating zone with its length moving along said first direction and said field lines penetrating said material generally along its direction of depth whereby said field lines form an angle-of incidence with said material at said location of maximum intensity of said field, and decreasing said angle of incidence where said intensity of said field is reduced from said maximum.

33. The method of claim 32 wherein the width of said material extends between said sides of said treating zone and said angle of incidence is reduced proceeding from the center of said treating zone to each side thereof.

34. The method of claim 32 further comprising transmitting unexpended microwave power from said treating zone to a terminating load to dissipate said power without reflection.

35. The method of claim 32 further comprising the step of passing air through said treating zone to remove moisture and cooking gas therefrom.

36. he method of claim 32 wherein said material comprises individual lots of bacon strips of convenience size with adjacent strips of each lot arranged in shingled relation and further comprising the step of collecting fat rendered from said bacon by gravitational force.

37. [n a method of heating a generally thin material, the steps comprising placing the material in a treating zone and transmitting a microwave field substantially in the TE, mode over said material while varying the angle of incidence of said field on said material as a function of the intensity of said field at the location of incidence to cause microwave energy to couple into said material according to a predetermined pattern along a direction transverse of the direction of power flow.

38. The method of claim 37 further comprising the step of continuously moving said material through said treating zone.

39. The method of claim 38 wherein the direction of movement of said material is along the direction of microwave power flow.

40. The method of claim 37 further comprising the step of passing air through said treating zone.

41. The method of claim 37 further comprising the step of dissipating all of the power transmitted through said treating zone without substantial reflection back into said zone.

42. In a system for heating a material with microwave energy, the combination comprising a waveguide applicator providing a heating chamber and having a pair of opposing broadwalls, one of said broadwalls defining an aperture elongated transverse of the direction of power flow for admitting passage of said material to said heating chamber, reject filter means including a waveguide having opposing broadwalls connected to said one broadwall of said applicator adjacent said aperture, and stub means in one broadwall of said reject filter means for rejecting the passage of microwave energy therethrough.

43. The apparatus of claim 43 wherein the broadwalls of said reject filter means comprise upper and lower broadwalls and wherein said stub means is in said upper broadwall, and further comprising means for moving said material through said reject filter means in proximity to said second broadwall thereof.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 I 597 I 565 Dated August 3 1971 Inventor(s) y M- Johnson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In claim 13: "broadwalls an elongated slot" should be "defines an elongated slot" Signed and sealed this 11th day of January 1972.

i (SEAL) jAttest:

i EDWARD M.FLETCHER, JR. ROBERT GOT'ISCHALK Attesting: Officer Acting Commissioner of Patents OHM PO-IOSO (10-69) USCOMM Dc 60376 P69 

1. A system for applying microwave energy to a material comprising waveguide means providing first and second spacedapart broadwall means arranged in opposing relation, excitation means including a source of microwave energy coupled to said waveguide means for energizing the same to provide an electric field of varying intensity extending between said first and second broadwall means, supporting means for supporting said material between said broadwall means and means for moving said material between said first and second broadwalls and along the direction of power flow through said waveguide, said waveguide means and said supporting means cooperating to vary the angle of incidence of said electric field on said material as the intensity of said electric field varies to thereby couple energy into said material along said direction of varying field intensity according to a predetermined pattern.
 2. The system of claim 1 wherein said supporting means supports said material in a generally flat disposition and wherein at least one of said first and second broadwall means defines a surface inclined relative to the disposition of said material in a direction transverse of the movement of said material.
 3. The system of claim 2 wherein said waveguide means includes a waveguide-heating chamber defined on a first pair of opposing sides by said first and second broadwall means and on a second pair of opposing sides by first and second sidewall each coupled between said first and second broadwall means, and wherein said excitation means energizes said waveguide heating chamber substantially entirely in the TE10 mode.
 4. The apparatus of claim 3 wherein said waveguide-heating chamber is elongated in a generally horizontal direction and each of said first and second broadwall means is symmetrically inclined about its longitudinal center to define a V-shape whereby the angle of incidence of the electric field lines between said broadwall means decreases as the intensity of said electric field decreases.
 5. The apparatus of claim 3 wherein said waveguide means further comprises input waveguide means interposed between said heating chamber and said excitation means for coupling power therebetween, said input waveguide means further defining an input aperture for receiving said material and permitting passage thereof to said heating chamber, and rejection filter means coupled to said input waveguide means adjacent said opening to prevent the escape of microwave energy therethrough.
 6. The system of claim 5 wherEin the lower broadwall of said heating chamber is V-shaped in transverse section having a low point at its transverse center and defines a slot extending in the center thereof along the direction of power flow whereby liquids rendered from said material during the application of microwave energy thereto are drained under gravity along said lower broadwall toward said slot.
 7. The system of claim 5 wherein said input means is a rectangular waveguide elbow-receiving power in a generally vertical direction and transmitting the same to said heating chamber in a generally horizontal direction, and wherein said aperture in said input means is defined in a broadwall thereof transverse to the flow of power therethrough.
 8. The system of claim 1 further comprising termination means coupled to said waveguide means downstream in the direction of power flow from a location at which said material moves between said first and second broadwalls whereby said termination means absorbs power not absorbed by said material to minimize reflected power.
 9. The system of claim 8 wherein said broadwalls of said waveguide means provide a heating chamber, and further comprising output waveguide means interposed between said waveguide-heating chamber and said termination means for coupling power therebetween, said output waveguide means further defining an output aperture for exiting said material from said heating chamber, and rejection filter means coupled to said output waveguide means to prevent the escape of microwave energy through said opening.
 10. The system of claim 8 wherein said termination means comprises an elongated termination waveguide section receiving microwave energy from said broadwalls, and conduit means within said termination waveguide section and extending longitudinally thereof for carrying water through the same.
 11. The system of claim 1 wherein said waveguide means comprises an elongated waveguide-heating chamber of rectangular cross section extending in a generally horizontal direction whereby said broadwalls are planar, and wherein said supporting means moves said material longitudinally through said heating chamber and supports said material in a disposition angularly inclined relative to said broadwalls over at least a portion of said material.
 12. The system of claim 1 wherein said waveguide means includes input waveguide means receiving power from said source and including a waveguide input elbow delivering said power in a generally horizontal direction, waveguide-heating chamber means receiving power from said input waveguide means and having upper and lower opposing broadwalls each defining a V-shape in cross section with the center being the lowest point of each, and output waveguide means receiving power from said heating chamber and including means for absorbing said power in termination.
 13. The system of claim 12 wherein said lower broadwall of said heating chamber broadwalls an elongated slot extending through its center in the direction of power flow.
 14. The system of claim 13 wherein said input elbow defines an opening for admitting passage of said material to said heating chamber, and further comprising wave guide rejection filter means at said opening to prevent the escape of microwave energy therethrough.
 15. The system of claim 12 wherein said side opposing section of said V-shaped broadwalls is planar and opposing pairs are separated by a distance less than one-half the free space wavelength of the excitation frequency.
 16. In a system for cooking separate lots of bacon strips of convenience size wherein the strips of each lot are in shingled relation, the combination comprising a waveguide applicator providing a heating chamber having upper and lower opposing broadwalls extending in a generally horizontal direction each longitudinal side of each of said broadwalls inclined upwardly about the longitudinal center line thereof, conveyor means supporting a plurality of lots of said bacon for carrying the same in a generally horizontal disposition through said heating chamber between said broadwalls with each individual strip extending transversely of said broadwalls, and source means for energizing said applicator substantially entirely in the TE10 mode.
 17. The system of claim 16 wherein said applicator further comprises input elbow means receiving power from said source for coupling said power to said waveguide-heating chamber, said input elbow receiving said power in a generally vertical direction and transmitting the same in a generally horizontal direction to said heating chamber and defining an aperture for admitting passage of said bacon to said chamber.
 18. The system of claim 17 further comprising rejection filter means adjacent said admitting aperture for preventing the passage of microwave energy therethrough.
 19. The system of claim 16 wherein the lower broad wall of said heating chamber defines an elongated slot extending longitudinally along the center line thereof for draining fat rendered from said bacon.
 20. The system of claim 19 further comprising means for circulating air through said heating chamber.
 21. The system of claim 20 further comprising an output wave guide elbow receiving power from said heating chamber for transmitting the same in a generally vertical direction and defining an aperture for admitting the egress of said bacon, and termination means receiving power from said output elbow and including a load for absorbing substantially all of said power without reflection.
 22. The system of claim 21 further comprising output rejection filter means coupled to output wave guide elbow adjacent said aperture for preventing the passage of said microwave energy therethrough.
 23. The system of claim 19 wherein said heating chamber is symmetrical about a vertical plane extending through the longitudinal center thereof to prevent the excitation of TEmo modes where m is an even integer.
 24. The system of claim 23 wherein the symmetrical half sections of said heating chamber are separateable to permit access to the interior thereof for cleaning and maintenance.
 25. In a system for applying microwave energy of a given wavelength to a material, the combination of a waveguide, and source means for energizing said waveguide in the TE10 mode, said waveguide providing first and second opposing broadwalls elongated in the direction of power flow through said waveguide and inclined relative to a surface of said material to vary the angle of incidence of the electric field on said surface as said electric field decreases in a direction transverse of the direction of power flow through said waveguide to thereby cause a more uniform heating of said material along a direction transverse of the direction of power flow.
 26. The system of claim 25 wherein said material has a generally horizontal disposition, and wherein opposing portions of said first and second broadwalls are parallel to each other and inclined relative to the horizontal whereby one of said broadwalls beneath said material will catch any liquid rendered therefrom and cause the same to be collected.
 27. In a system for heating material having a thickness which is generally uniform along a first direction, the combination of means for conveying said material in a second direction perpendicular to said first direction, and waveguide means having opposing broadwall means for generating an electric field varying in intensity in said first direction and for transmitting microwave power in said second direction, said waveguide means cooperating with said conveying means to orient said electric field lines to vary the angle of incidence of said lines on the surface of said material as said intensity varies to thereby cause a more uniform heating of said material along said first direction even though the intensity of said field decreases along said first direction.
 28. A method of heating a material having a generally uniform depth along one diRection, comprising propagating microwave power through a waveguide in the direction of elongation of its opposing broadwalls, and passing said article between said broadwalls in the direction of elongation thereof while establishing the electric field intensity lines to vary their angles of incidence on the surface of said article in a plane perpendicular to said direction of elongation to induce uniform heating of said article along said one direction.
 29. In a method of heating a material, the steps of passing said article in one direction through a waveguide, propagating microwave power through said waveguide along said direction, and establishing the electric field of said power to vary its angle of incidence on the surface of said article to cause uniform heating of said article along a second direction transverse of said one direction.
 30. The method of claim 29 further comprising the step of passing air over said material as it is heated with said microwave energy.
 31. The method of claim 29 further comprising continuously draining liquid rendered from said material during heating.
 32. A method of heating a material having length, width and generally uniform depth comprising transmitting microwave power in a first direction through a treating zone, said microwave power generated by an electric field excited at a predetermined frequency and the intensity of said field varying from a minimum at the side of said treating zone to a maximum intermediate said sides, moving said material through said treating zone with its length moving along said first direction and said field lines penetrating said material generally along its direction of depth whereby said field lines form an angle of incidence with said material at said location of maximum intensity of said field, and decreasing said angle of incidence where said intensity of said field is reduced from said maximum.
 33. The method of claim 32 wherein the width of said material extends between said sides of said treating zone and said angle of incidence is reduced proceeding from the center of said treating zone to each side thereof.
 34. The method of claim 32 further comprising transmitting unexpended microwave power from said treating zone to a terminating load to dissipate said power without reflection.
 35. The method of claim 32 further comprising the step of passing air through said treating zone to remove moisture and cooking gas therefrom.
 36. The method of claim 32 wherein said material comprises individual lots of bacon strips of convenience size with adjacent strips of each lot arranged in shingled relation and further comprising the step of collecting fat rendered from said bacon by gravitational force.
 37. In a method of heating a generally thin material, the steps comprising placing the material in a treating zone and transmitting a microwave field substantially in the TEmo mode over said material while varying the angle of incidence of said field on said material as a function of the intensity of said field at the location of incidence to cause microwave energy to couple into said material according to a predetermined pattern along a direction transverse of the direction of power flow.
 38. The method of claim 37 further comprising the step of continuously moving said material through said treating zone.
 39. The method of claim 38 wherein the direction of movement of said material is along the direction of microwave power flow.
 40. The method of claim 37 further comprising the step of passing air through said treating zone.
 41. The method of claim 37 further comprising the step of dissipating all of the power transmitted through said treating zone without substantial reflection back into said zone.
 42. In a system for heating a material with microwave energy, the combination comprising a waveguide applicator providing a heating chamber and having a pair of opposing broadwalls, one of said broadwalls defining an aperture elongated transverse of the diRection of power flow for admitting passage of said material to said heating chamber, reject filter means including a waveguide having opposing broadwalls connected to said one broadwall of said applicator adjacent said aperture, and stub means in one broadwall of said reject filter means for rejecting the passage of microwave energy therethrough.
 43. The apparatus of claim 43 wherein the broadwalls of said reject filter means comprise upper and lower broadwalls and wherein said stub means is in said upper broadwall, and further comprising means for moving said material through said reject filter means in proximity to said second broadwall thereof. 